The invention relates to a method of conjoint detection of a set of CDMA codes used in mobile telephony.
The Universal Mobile Telecommunications System (UMTS) is the new mobile telephony standard intended to replace the GSM standard. The first commercial applications of the new standard should see the light of day between year 2000 and year 2005.
Compared to the aforementioned GSM standard, the main improvements are as follows : although in the GSM each base transceiver station can dialogue only with eight mobile stations, with a maximum bit rate of 13 kbit/s for each mobile station, the UMTS increases the number of users and can allocate each mobile station resources for transmitting information at bit rates from 4 kbit/s to 2 Mbit/s, depending on the nature of the information transmitted, such as voice, data, video telephony, hi-fi audio or video.
With this aim in view, two solutions have been adopted in Europe with regard to the multi-user ratio access technique, namely the wideband-CDMA technique in Frequency Division Duplex (FDD) mode and the TDMA/CDMA technique with conjoint detection in Time Division Duplex (TDD) mode.
A major drawback of the TDMA/CDMA technique in the TDD mode is the great complexity of conjoint detection of CDMA codes, conjoint detection being defined as simultaneous detection of all CDMA codes received at various antennas and simultaneously active, unlike the rake receiver habitually used in CDMA, which detects one code at a time, the other codes being regarded as jamming signals.
The object of the present invention is to remedy the aforementioned drawbacks by using a simplified method of conjoint detection of CDMA codes.
One particular object of the present invention is to provide a method of conjoint detection of CDMA codes in which a successive process is used, each successive call request leading to the application of simplified conjoint detection of a symbol for each CDMA code simultaneously active, the next call request conjointly detecting all the symbols of all the CDMA codes simultaneously active.
The method of conjoint detection of a set of K simultaneously active CDMA codes received at Ka antennas of a mobile transceiver applies to codes transmitted via a multipath stationary linear transmission channel with Gaussian additional white noise and satisfying the equation e=A.d+n where A is the transfer matrix of said multipath stationary linear channel, d is the set of symbols of the codes to be transmitted, n is the additional noise vector and e is the set of received samples. An estimate xcex4 of the set d of symbols of the codes is obtained by pseudo-inversion of said transfer matrix of the form xcex4=(A+.A)xe2x88x921.A+.e where the symbol+denotes the transposition and complex conjugation operator. A characteristic feature of the method according to the invention is that, for a set of transfer sub-matrices AS corresponding to a number S of rows of blocks of said transfer matrix A, it determines a set of equalising filters for successively estimating all the symbols transmitted by the CDMA codes from a particular number of received samples eS.